Generally, a ceramic capacitor includes a chip-shaped sintered body and a pair of electrodes formed at two opposite sides thereof. In case of a multi-layer ceramic capacitor, the sintered body is generally made of alternately laminated dielectric layers and internal electrodes. Every two neighboring internal electrodes face each other through a dielectric layer disposed therebetween, and are electrically coupled to different external electrodes, respectively.
The dielectric layer is formed of a reduction-resistant dielectric ceramic, which includes ceramic grain primarily composed of BaTiO3, and an additive having a glass component serving to combine the ceramic grains. The internal electrodes are made of sintered conductive paste primarily composed of, e.g., Ni metal powder. Sintering as defined herein represents a process in which individual particles are densified through modification and bonding below melting point thereof to have a poly-crystalline structure in a shape of mass.
The sintered body is made by performing removal of binder from alternately laminated ceramic green sheets and internal electrode patterns, sintering in a non-oxidizing atmosphere at a high temperature of about 1200˜1300° C., and thereafter re-oxidizing under a mild oxidation condition.
If a ratio of Ba/Ti(A/B) of BaTiO3 contained in the dielectric layers is equal to or less than 1.000 and sintering is performed in a reducing atmosphere, the sintered product does not function as a capacitor, since the constituents of the dielectric ceramic become semi-conductive during sintering and thus insulating properties thereof is deteriorated. To improve reduction-resistant properties of the dielectric ceramic, a ratio A/B of BaTiO3 is required to be greater than 1.000. For making A/B greater than 1.000, it has been proposed to put an A-site component such as barium, strontium, and calcium greater than a stoichiometric ratio.
However, when sintering the dielectric ceramic having thus enhanced reduction-resistant characteristics, the A-site component of the perovskite crystal structure diffuses to grain boundaries so that the ratio A/B of ceramic grains becomes lowered. Therefore, reduction-resistance of the dielectric ceramic is deteriorated and oxygen deficiencies increase, resulting in a lifetime, i.e., a reliability, of a ceramic capacitor to be degraded.